1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method for the same, and more particularly to a semiconductor device suitable for a liquid crystal display device, an image sensor and so on, and a manufacturing method for the same.
2. Description of Related Art
As the method which drives a liquid crystal display device to display an image, there are conventionally known a simple matrix method and an active matrix method. In the simple matrix method, a display pixel is driven by the voltage which is applied between a scan electrode and a signal electrode provided in an orthogonal manner to the scan electrode. On the other hand, in the active matrix method, a switching element is provided in an intersection point of a scan electrode and a signal electrode and each of display pixels is driven by the switching element independently. This active matrix method is mainly classified into two groups based on a kind of switching element to be used; one is a 2-terminal method using a diode and a 3-terminal method in which a thin film transistor element including an active layer and formed of amorphous silicon (a-Si) or polysilicon is used. Especially, since the liquid display device using the thin film transistor has a high resolution, a high aperture ratio and a high gradation display function, the image of high quality can be obtained (For example, see Japanese Laid Open Patent Disclosure (JP-A-Heisei 5-210116)).
FIG. 1 is a plan view illustrating unit pixels of an active matrix array substrate of a conventional liquid crystal device which uses a thin film transistor formed of polysilicon. FIG. 2 is a cross sectional view of a unit pixel of the thin film transistor taken along the line A-A' in FIG. 1. In the FIGS. 1 and 2, a reference numeral 1 denotes a quartz glass substrate. A reference numeral 2 denotes a data line which is composed of aluminum. The data line 2 is formed above the quartz glass substrate 1. A reference numeral 3 denotes a gate line which is composed of aluminum. The gate line 3 is provided to be orthogonal to the data line 2. A thin film transistor 4 is formed in correspondence to each of intersection points between the data lines 2 and the gate lines 3. Also, a reference numeral 11 is a source, 12 is a drain, and 13 is an active layer. A reference numeral 14 is a gate oxide film, 15 is a gate electrode, and 16 is a pad electrode which is composed of aluminum. A reference numeral 17 is a LOCOS film (Local Oxidation of Silicon film) which is formed on the quartz glass substrate 1. A reference numeral 18 is an interlayer insulating film which is composed of SiO.sub.2 and which is formed on the LOCOS film 17. A reference numeral 19 is a transparent pixel electrode which is composed of or ITO (indium-tin-oxide) or In.sub.2 O.sub.3 (indium oxide) in which Sn ions are doped. The transparent pixel electrode 19 is formed on the pad electrode 16 and the interlayer insulating film 18. The source 11 is electrically connected to the data line 2 and the drain 12 is electrically connected to the transparent pixel electrode 19 through the pad electrode 16. Note that reference numerals 21 and 22 are the first and second contact holes formed in the interlayer insulating film 18.
In the liquid crystal device, the thin film transistor 4 is selectively driven in response to a signal which is inputted from the gate line 3. The voltage to drive the liquid crystal is applied to the transparent pixel electrode 19 from the data line 2 when the thin film transistor 4 is in the on state. On the other hand, when the thin film transistor 4 is in the off state, a pixel potential is maintained by a pixel capacitance which is formed between the transparent pixel electrode 19 and a common electrode (not illustrated) which is formed on the second substrate opposing to the transparent pixel electrode, so that a display state is maintained.
Next, the manufacturing method of the above-mentioned liquid crystal device will be described with reference to FIGS. 3A to 3C.
First, a semiconductor layer 31 which is composed of polysilicon on the quartz glass substrate 1 is formed as shown in FIG. 3A. A silicon nitride film is formed to cover a portion 31b of the semiconductor layer 31 where a thin film transistor is to be formed. A portion 31a of the semiconductor layer 31 which is not covered by the silicon nitride film is oxidized by annealing in oxygen (O.sub.2) ambience to forms the LOCOS film 17.
Next, the silicon nitride film is removed. Subsequently, a gate oxide film 14 composed of SiO.sub.2 is formed on the semiconductor layer portion 31b and then a gate electrode 15 which is composed of polysilicon and which contains P ions of about 10.sup.20 cm.sup.-3 is formed. Then, the gate oxide film 14 and the gate electrode 15 are patterned. Subsequently, P ions of about 10.sup.20 cm.sup.-3 are implanted in self-alignment with the gate electrode by an ion implantation method such that the P ions are doped in the area of the semiconductor layer portion 31b where the gate electrode 15 is not formed. As a result, the source 11 and the drain 12 are formed. The semiconductor layer portion 31b where the P ions are not implanted functions as the active layer 13.
Next, as shown in FIG. 3B, the interlayer insulating film 18 is formed to cover the whole surface of the quartz glass substrate 1 on which the thin film transistor 4 is formed. Portions of the interlayer insulating film 18 above the source 11 and the drain 12 are removed by an etching method to forms the first contact hole 21 and the second contact hole 22, respectively. Subsequently, the data line 2 which is electrically connected to the source 11 and the pad electrode 16 which is electrically connected to the drain 12 are formed at the same time by a sputtering method.
Next, as shown in the same FIG. 3C, the transparent pixel electrode 19 is formed by the sputtering method in ambience of mixed gas of O.sub.2 and Ar to cover a part of each of the pad electrodes 16 and the interlayer insulating films 18 is formed.
Thereafter, an orientating film is formed on the transparent pixel electrode 19 to perform orientation processing. The second substrate on which a transparent electrode has been formed is arranged in such a manner that this transparent electrode opposes to the above-mentioned transparent pixel electrode 19. Subsequently, after these substrates are sealed, the liquid crystal is injected.
In this liquid crystal device, since the pad electrode 16 is formed between the drain 12 and the transparent pixel electrode 19, even if the interlayer insulating film 18 is formed between the drain 12 and the transparent pixel electrode 19 to have a thick thickness, it becomes possible to prevent the transparent pixel electrode 19 from being broken due to a step at the opening of the second contact hole 22.
On the other hand, as the structure which can prevent aluminium as electrode material from diffusing in silicon, there is widely known the structure in which a barrier metal film obtained by laminating a metal film of Ti and a film of Ti.sub.3 N.sub.4 is formed between the aluminum electrode and the silicon substrate (see, for example, Japanese Laid Open Patent Disclosure (JP-A-Heisei 1-235334)).
Next, the method of forming the barrier metal film will be described with referring to FIGS. 4A to 4C.
First, as shown in FIG. 4A, a titanium film 44 having the thickness of 20 nm is formed to have ohmic contact by a sputtering method on a silicon substrate 43 on which an insulating film 41 has been formed and moreover a contact hole 42 has been formed in the insulating film 41.
Next, as shown in FIG. 4B, a titanium nitride film 45 having the thickness of 80 nm is formed on the titanium film 44 using a reactive sputtering method by applying an RF (high frequency) substrate bias.
Next, as shown in FIG. 4C, an Al--Si alloy film 46 having the thickness of 600 nm is formed on the titanium nitride film 45 by the sputtering method.
In this way, by forming the barrier metal film which is composed of the lamination structure of the titanium film 44 and the titanium nitride film 45, the diffusion of aluminium (Al) to silicon (Si) can be prevented.
By the way, in the above-mentioned conventional liquid crystal device, there is a problem in that pixel defect is generated because the contact resistance between the transparent pixel electrode 19 and the pad electrode 16 becomes high. The reason is that because the formation of the transparent pixel electrode 19 is performed in a mixed gas ambience of O.sub.2 and Ar, the oxide film is formed as a high resistance layer on the surface of the pad electrode 16, resulting in the high contact resistance. If this contact resistance becomes high, even if the thin film transistor 4 is set to the ON state, a voltage drop is caused by the contact section between the transparent pixel electrode 19 and the pad electrode 16 so that it becomes not possible to write data in the transparent pixel electrode 19.
On the other hand, in the structure in which the barrier metal film is formed, when the barrier metal film is formed between the aluminum electrode and the transparent electrode, there is a problem in that the contact resistance between the barrier metal film and the transparent electrode becomes high. The reason is that the barrier metal film and the transparent electrode can not be continuously formed because the plane pattern shape of the barrier metal and that of the transparent electrode are different from each other. Accordingly, after the barrier metal film is formed, the wafer is necessarily exposed to atmosphere so that the barrier metal film surface is oxidized. Also, because the titanium nitride film 45 has the cylindrical structure, O.sub.2 is taken into the grain boundary when the titanium nitride film 45 is exposed to the atmosphere. Generally, the method in which the titanium nitride film 45 is exposed to the atmosphere to take O.sub.2 into the grain boundary is a very useful method because it becomes possible to prevent aluminum from diffusing along the grain boundary. However, there is a problem in that the contact resistance increases because the titanium nitride film 45 becomes high resistance.